1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method. Specifically, the present invention relates to a technique for suppressing an image defect attributed to an error in conveyance of a printing medium.
2. Description of the Related Art
A general configuration of an inkjet printing apparatus includes a conveying unit for conveying a printing medium along a predetermined conveying path, and a printing head in which nozzles are arranged for ejecting the inks on the printing medium thus conveyed. The inkjet printing apparatus performs a printing by an alternate series of an operation for ejecting the inks from the nozzle while causing the printing head to move in the main scanning direction different from a direction in which the nozzles are arranged and an operation for causing the conveying unit to convey the printing medium in a sub-scanning direction orthogonal to the main scanning direction.
In this respect, there is a printing apparatus of a type which has two conveying units (hereinafter referred to as an upstream conveying unit and a downstream conveying unit) respectively arranged upstream and downstream of a position (a printing position) where the printing apparatus performs a printing by use of the printing head. The upstream conveying unit and the downstream conveying unit are involved in a printing medium conveyance operation including supplying a printing medium to the printing position and discharging the printing medium from the printing position. In general, the upstream conveying unit includes a conveying roller, whereas the downstream conveying unit includes a discharge roller. A set of pinch rollers is provided to the conveying roller to hold a printing medium between the conveying roller and the pinch rollers while elastically biased by a pressing member such as a spring, whereas a set of spur rollers is provided to the discharge roller to hold the printing medium between the discharge roller and the spur rollers while elastically biased by a pressing member such as a spring. Specifically, the upstream conveying unit and the downstream conveying unit are respectively configured of a roller pair of the conveying roller and the set of pinch rollers, and a roller pair of the discharge roller and the set of spur rollers.
While the inkjet printing apparatus of this type is making a printing in the front end portion of the printing medium, the printing medium is conveyed by the upstream conveying unit only. Further, while the inkjet printing apparatus of this type is making a printing in the rear end portion of the printing medium, the printing medium is conveyed by the downstream conveying unit only. For this reason, the inkjet printing apparatus of this type tends to convey the printing medium with a lower accuracy. This kind of conveying condition takes place when the inkjet printing apparatus is making a printing (termed as a margin-less printing) on the front or rear end portion of the printing medium with no margin left there, or a printing near the front or rear end of the printing medium even with a margin left there. If the printing medium is conveyed at an insufficiently short distance between two consecutive main scans, an image formed during the main scan overlaps another image formed during the subsequent main scan on the printing medium, so that what is termed as a black stripe occurs. By contrast, if the printing medium is conveyed at an excessive distance between two consecutive main scans, an image formed during the first main scan is distanced from another image formed during the subsequent main on the printing medium scan, so that what is termed as a white stripe occurs.
In addition to these types of image defects stemming from the reduced accuracy with which a printing medium is conveyed, another type of image defect stems from a printing medium which is not held by both the upstream conveying unit and the downstream conveying unit. While a printing medium is being held by either of the two conveying units, the distance between the printing head and the printing medium (hereinafter referred to as a “medium-to-head distance”) varies to a non-negligible extent because the printing medium curls. As a result, the printing medium is likely to be placed in an unstable condition. The printing head is designed to perform printing scans while ejecting inks on the printing medium at timings corresponding to a predetermined medium-to-head distance maintained by both the upstream and downstream conveying units. Thus, inks ejected at appropriate timings form dots, and these dots are arranged in appropriate pitches on the printing medium to form an image. For this reason, if the medium-to-head distance varies while a printing is being performed, or if the medium-to-head distance varies to a large extent in a printing swath, dots are positioned on the printing medium unstably. This causes image defects, including a white stripe, a black stripe and a granular impression.
In this respect, a scheme of reducing a conveying distance of a printing medium at one time is effective for increasing the conveyance accuracy of printing medium. However, a uniform reduction of the one-time conveying distance throughout the printing medium reduces printing throughput. To avoid a reduction in the throughput, a one-time conveyance distance for the front and rear end portions of a printing medium is usually set shorter than a one-time conveyance distance for a central portion of the printing medium. This is because the front and rear end portions thereof are conveyed with the specifically reduced accuracy, and because the central portion thereof is held by both the upstream and downstream conveying units. If, however, the front and rear end portions thereof are conveyed at a shorter distance at one time than the central portion thereof while a range of nozzles within a nozzle arrangement range of the printing head is used to perform the printing without any change, the density becomes uneven because there is a difference in the number of printing scans in a unit of printing area between the front and rear end portions of the printing medium, and the central portion thereof.
With this taken into consideration, particularly a serial type of printing apparatus, emphasizing its image quality, takes the following step against an image defect which would otherwise occur while making a printing in the front and rear end portions of a printing medium. For example, Japanese Patent Laid-Open No. 2004-98668 describes a method using a printing head in which the nozzles are arranged with a density corresponding to a printing density in the same direction as a printing medium is conveyed. In this method, a printing is performed on the front and rear end portions of a printing medium by using only a restricted part of the nozzles as a using range of nozzles (nozzle-use range) while the printing medium is conveyed at shorter distance at one time. Even when a conveyance accuracy of the printing medium decreases, the foregoing method is capable of reducing the conveyance error of the printing medium by decreasing the distance at which the printing medium is conveyed at one time. In addition, the method decreases the printing swath for each main scan, and is thus capable of suppressing the adverse affect of the variation in the medium-to-head distance. Furthermore, the method narrows down the pitch of image forming areas connected to each other and printed by the two consecutive main scans a connecting portion between image forming areas respectively corresponding to each two consecutive main scans. As a result of all of the foregoing effects, the method brings about an effect of making white and black stripes unobvious.
A combined use of the foregoing method and what is termed as a multi-pass printing method is more effective against an image defect which would otherwise occur when a printing is performed in the front and rear end portions of a printing medium. In addition, an effect against the granular impression is also expected of the combined use. The multi-pass printing method is for forming an image on a printing medium by making a printing in each printing area on the printing medium by multiple main scans while conveying the printing medium at a distance shorter than the printing swath between two consecutive main scans. In the case of the multi-pass printing method, for each main scan, it is predetermined which nozzles are used to eject inks and which nozzles are not, that is, which dots are allowed to be printed and which are not. To put it the other way, a print allowing rate is predetermined for each main scan. Furthermore, mask patterns are applied in order that multiple main scans should be mutually complementary to perform a printing. In this respect, Japanese Patent Laid-Open No. 2005-231353 describes a technique for changing mask patterns to be applied to the nozzle-use range to perform a printing on the rear end portion of the printing medium when the printing proceeds from the central portion to the rear end portion. In addition, a proposal has been made that an image defect should be reduced by using mask patterns, whose print allowing rates have a larger gradient for the nozzle-use range used for the front and rear end portions of a printing medium than that for the nozzle-use range used for the central portion thereof (Japanese Patent Laid-Open No. 2006-96031).
On the other hand, there are apparatuses of a type using a method of making a printing on a printing medium by use of a printing head in which the nozzles are arranged in a density lower than the printing density while interpolating the printing density in the sub-scanning direction with multiple main scans. To carry out this interpolation, the printing medium is conveyed between each two consecutive main scans so that the nozzles can be positioned in a line having no dots formed by the previous main scan. This method is termed as an interlace printing method. Among apparatuses using this method, some further employs the method for making a printing in the front and rear end portions of the printing medium with a smaller nozzle range while conveying the printing medium at a shorter distance between each two consecutive main scans (for example, Japanese Patent Laid-Open No. 11-291506 (1999)).
Application of these techniques reduces an image defect which would otherwise occur when a printing is performed in the front end portion of the printing medium held and conveyed only by the upstream conveying unit, and when a printing is performed in the rear end portion of the printing medium held and conveyed only by the downstream conveying unit.
However, the inventors of the present invention have found that a mere application of the techniques disclosed in the foregoing documents rather performs the printing density uneven in some cases. Descriptions will be provided hereinbelow for this finding.
In some cases, an error in conveyance of a printing medium suddenly takes place due to an impact which occurs when the front end of the printing medium goes into the downstream conveying unit (or plunges into a nipping part between the discharge roller and the spur rollers). In addition, in some cases, another error in conveyance of the printing medium suddenly takes place due to an impact which occurs when the rear end of the printing medium comes out of the upstream conveying unit (or is released from the nipping part between the conveying roller and the pinch rollers).
Here, let us assume that a first nozzle array for ejecting an ink of a first color and a second nozzle array for ejecting an ink of a second color whose tone is different from that of the first color are used. Under this condition, if the techniques disclosed in the foregoing documents are merely applied, the nozzle-use ranges are restricted to certain parts in the first and second nozzle arrays at the same positions relative to the whole nozzle arrays. For this reason, a sudden conveyance error uniformly displaces an image forming area printed with the first nozzle array and an image forming area printed with the second nozzle array, from their ideal positions. This brings about a problem that the sudden conveyance error varies density or brightness obviously.